Heretofore, there are two methods used in branching filters for mobile telephones in the 800 MHz band area.
In the first method, as shown in FIG. 4, an input signal from antenna 1 is fed through branching circuit 2, which is a transmission line composed from a coil and a condenser or the like, to receiver di-electric bandpass filter 11 consisting of high di-electric loss ceramic material having an elimination band area at the transmitting frequency zone. The output signal Rx from filter 11 is fed to a receiver. A transmitting signal Tx is fed to a similar di-electric bandpass filter 10 having an elimination band area at the receiving frequency zone and to antenna 1 through branching filter 2. However, filters 10, 11 must be large to satisfy the required characteristics of insertion loss and damping factor from the method used in communication equipment because these filters 10, 11 are cubic cavity resonators. That is, the ratio of the damping factor determines the necessary sections of a resonator and the allowable insertion loss of a bandpass filter determines the capacity of the resonators. In branching filters presently used, the insertion loss of di-electric bandpass filter 11 at the transmitting side is 3 dB, and the insertion loss of di-electric bandpass filter 10 at the receiving side is 2 dB.
In a second method, shown in FIG. 5, branching filter 2 is a combination of a receiving side SAW (Surface Acoustic Wave) bandpass filter 7, 9 having a damping characteristic curve as shown in FIG. 6(a) and a transmitting side SAW band stop filter 6 having a damping characteristic curve shown in FIG. 6(b). The bandpass insertion loss of SAW bandpass filters 7, 9 at the receiving side are 4-6 dB, and the bandpass insertion loss of SAW band stop filter 6 at the transmitting side is 1.5-2 dB.
This method has a further disadvantage over the first method because losses at filters 7, 9 result in a lower noise function of the receiver. Therefore, in the second method, to relieve this disadvantage, amplifier 8 having a low noise and high gain GaAs FET transistor (Gallium Arsenide Field Effect Transistor) must be interposed between filters 7, 9.
However, while a GaAs FET transistor is better under lower voltage operation than a bipolar transistor, it consumes large amounts of electric current. Therefore, this type of branching filter is a disadvantage for portable communication machines which require low consuming current.
Moreover, while a SAW filter is small, such as 0.2 cc, it needs a low noise amplifier 8 in the receiving circuit, and additional circuits, such as a bias circuit and a matching circuit, hinder heat radiation of SAW filter 6 at the receiving side, and the second method requires 70-80% of the volume of the first method.